It is desirable in many applications, for example, in fuel control for automotive engines to accurately measure the mass airflow rate of a stream of air. A particular arrangement for making such a measurement utilizes a hot wire or a hot film subject to the air stream. An example of a hot film apparatus is shown in the U.S. patent to Shih et al. U.S. Pat. No. 4,433,576. In operation an element which comprises an electrical resistor or which contains a resistor is electrically heated to a temperature higher than the ambient, which temperature is constant for a given ambient air temperature. The cooling effect of the air stream on the hot element is a function of the mass airflow rate past the element. Since the cooling effect depends upon the ambient temperature as well as the difference between the element temperature and the ambient temperature, it is common practice to utilize a compensating circuit containing a reference temperature sensor to accurately compensate for temperature changes. In this manner the heat transfer rate from the hot element by convection very accurately correlates to the mass airflow rate. This value is measured by maintaining the element at the prescribed temperature by electrically heating the element and measuring the power required for such maintenance. It is assumed that the maintenance power is equal to that lost by thermal convection. That assumption, however, introduces an error since another mechanism for energy transfer from the hot element is radiation loss to cooler surroundings. The radiation loss is independent of airflow and at high airflow rates is insignificant. When, however, low airflow rates are being measured the component of thermal loss by radiation can be a significant portion of the total thereby introducing an undesirable error into the airflow measurement.
The net radiated power from a surface can be approximated by the equation Q=.epsilon..sigma. A(T.sub.s.sup.4 -T.sub.a.sup.4) where .epsilon. is the emissivity of the surface, .sigma. is the Stefan-Boltzman constant, A is the area of the heated surface, T.sub.s is the surface temperature and T.sub.a is the ambient temperature. From this equation it can be seen that the net radiated power Q which represents an error in the airflow measurement can be minimized by minimizing the emissivity of the surface.